The present invention relates to localized deep-grid semiconductor structures and a process for manufacturing same. It relates more particularly to an application to diodes capable of being disabled by field effect.
In different types of discrete or integrated semiconductor components, such as high-power bipolar transistors, gate turn-off thyristors, junction power vertical field-effect transistors and others, a localized deep layer in the form of a grid is disposed inside the semiconductor wafer in a plane parallel to that of the principal faces of this wafer.
Such deep layers are currently designated in the technique by the expression "buried grids" because of the conventional manufacturing process of the prior art by which they were obtained. In fact, starting with a semiconductor substrate, this process includes ion implanting therein dopant atoms of a type of conductivity corresponding to that of the grid which it was desired to obtain, then in forming above this substrate one or more epitaxial layers in which one or more diffusions are possibly formed. Such a structure is shown schematically in the accompanying FIG. 1. There can be seen therein the substrate 1 overlaid by an epitaxial layer 2 in which is formed a diffused zone 3. The buried layer or grid 4 is present at the limit between substrate 1 and epitaxial layer 2. Then, a contact is provided between the upper surface of the semiconductor wafer and the buried grid 4 by means of a deep diffusion 5. In numerous practial applications, grid 4 must have an extremely fine and well-defined pitch. Furthermore, this grid generally serves for turning off the semiconductor device. It is then necessary for the transverse resistivity of the grid layer to be as small as possible. Its doping level must then be very high. This high doping level makes it difficult to control with great accuracy the extent of the diffusion from the initially implanted layer so that the mesh of the grid do not close up. The control of this diffusion is very delicate to achieve particularly because parasite diffusions of the implanted dopant may occur during formation of the epitaxial layer.
These drawbacks are all the more noticeable since, in numerous devices, attempts have been made to obtain in a practical way grid mesh dimensions of the order of a few microns. Furthermore, due to the very fact of using an epitaxial layer deposited on a substrate, the presence of defects at the epitaxial layer-substrate interface causes an excessive leak current from the grid junction which interferes with the proper operation of the device under satisfactory conditions.
Thus, an object of the present invention is to provide a novel deep-grid semiconductor device structure which palliates the structural or manufacturing drawbacks of the buried-grid devices of the prior art.
Another object of the present invention is to provide such a structure of the diode type capable of being disabled.
Another object of the present invention is to provide a novel process for manufacturing a deep-grid semiconductor device.